1. Field of the Invention
The present invention relates to an on-time control module and an on-time control method, and more particularly, to an on-time control module and on-time control method for compensating and stabilizing a switching frequency.
2. Description of the Prior Art
Power supply related devices are a crucial part in modern information technology. Among all kinds of power supply devices, DC-DC switching regulators have gained wide popularity, and a main function of the DC-DC switching regulators is to provide a stable direct current (DC) power to electronic components.
Please refer to FIG. 1, which is a schematic diagram of a fixed on-time DC-DC switching regulator 10 according to the prior art. The DC-DC switching regulator 10 provides a stable output voltage Vout to a load Ld, and includes an upper-gate switch 100, a lower-gate switch 102, an on-time control module 104, a comparator 106, an inductor L, a capacitor C and an invertor INV. The upper-gate switch 100, the lower-gate switch 102, the inductor L and the capacitor C form a power-stage circuit 108 to output the output voltage Vout to the load Ld. The on-time control module 104 outputs a control signal Con with an on-time Ton to control on/off operations of the upper-gate switch 100 and the lower-gate switch 102.
In short, whenever the output voltage Vout is lower than a reference voltage Vref, the comparator 106 would output a high-level comparison result Com to control the on-time control module 104 to trigger an on-time Ton in the control signal Con (during which the control signal Con is at the high-level), so as to turn on the upper-gate switch 100 and turn off the lower-gate switch 102. This in turn causes a system voltage Vin to transfer electric power to the inductor L via the upper-gate switch 100, outputting a load charging current IL to charge the capacitor C. As a result, the output voltage Vout (a voltage across the capacitor C) outputted to the load Ld increases until the output voltage Vout becomes greater than the reference voltage Vref, at which point the comparison result Com switches to a low level. Next, after the on-time Ton, the control signal Con switches back to the low-level, causing the upper-gate switch 100 to cut off and the lower-gate switch 102 to conduct, such that the output voltage Vout starts to decrease until it becomes lower than the reference voltage Vref. Then, the above-mentioned process is repeated. As such, the DC-DC switching regulator 10 is capable of controlling the on/off operation of the upper-gate switch 100 with a stable switching frequency Fsw, thereby regulating the electric power transmitted to the load Ld and maintaining stability in the output voltage Vout.
Moreover, the DC-DC switching regulator 10 often further includes a pulse width modulation (PWM) circuit, which is capable of modulating the control signal Con according to a configured duty D, to obtain the desired switching frequency Fsw. In more detail, please refer to FIGS. 2A and 2B, which are schematic diagrams of the load current IL and a voltage VL of the inductor L shown in FIG. 1 under ideal conditions. Under the ideal conditions, assuming that the upper-gate switch 100 and the lower-gate switch 102 have negligible conduction resistances when turned on, and that the load current IL has equal rising and lowering amplitudes, it is possible to obtain a relationship (Vin−Vout)*D*TS=Vout*(1−D)*TS, and therefore
  D  =            Vout      Vin        .  Under actual and non-ideal conditions, assuming that the upper-gate switch 100 and the lower-gate switch 102 have an equal conduction resistance Rds, and that the load current IL has equal rising and lowering amplitudes, the following may be obtained:[(Vin−IL*Rds)−Vout]*D*TS=(Vout+IL*Rds)*(1−D), and therefore
  D  =                              V          out                +                  IL          *          Rds                            V        in              .  Under this condition,
      Ton    =                  1                  F          sw                    *      D        ,and therefore if the on-time Ton is constant, then the duty D may be suitably configured to obtain the required switching frequency Fsw.
However, in reality, when the on-time control module 104 triggers the on-time Ton in the control signal Con, a length of the on-time Ton also varies under an effect of the load current IL and the conduction resistance Rds. Therefore, a fixed on-time Ton cannot be obtained with a fixed duty D to stabilize the switching frequency Fsw. Namely, the switching frequency Fsw would vary according to the load current IL and the conduction resistance Rds.
To stabilize the switching frequency Fsw, the conventional DC-DC switching regulator 10 further includes a feedback compensation unit coupled between the control signal Con and the on-time control module 104, for performing integration operation on the control signal Con, and then comparing the integration result with a reference voltage to adjust the length of the on-time Ton of the on-time control module 104, so as to stabilize the switching frequency Fsw. However, this method requires a large capacitor in the feedback compensation unit for performing the integration operation, resulting in an excessive circuit layout area. Hence, it is necessary to improve over the prior art.